Add automatic periodic reseeding

include/polarssl/hmac_drbg.h

@@ -51,18 +51,22 @@ extern "C" {
 
 /**
  * HMAC_DRBG context.
- * TODO: reseed counter.
  */
 typedef struct
 {
-    md_context_t md_ctx;
+    /* Working state */
-    unsigned char V[POLARSSL_MD_MAX_SIZE];
+    md_context_t md_ctx;                    /*!< HMAC context       */
-    unsigned char K[POLARSSL_MD_MAX_SIZE];
+    unsigned char V[POLARSSL_MD_MAX_SIZE];  /*!< V in the spec      */
+    unsigned char K[POLARSSL_MD_MAX_SIZE];  /*!< Key in the spec    */
+    int reseed_counter;                     /*!< reseed counter     */
 
+    /* Administrative state */
     size_t entropy_len;         /*!< entropy bytes grabbed on each (re)seed */
     int prediction_resistance;  /*!< enable prediction resistance (Automatic
                                      reseed before every random generation) */
+    int reseed_interval;        /*!< reseed interval   */
 
+    /* Callbacks */
     int (*f_entropy)(void *, unsigned char *, size_t); /*!< entropy function */
     void *p_entropy;            /*!< context for the entropy function        */
 } hmac_drbg_context;
@@ -128,14 +132,25 @@ void hmac_drbg_set_prediction_resistance( hmac_drbg_context *ctx,
 
 /**
  * \brief               Set the amount of entropy grabbed on each reseed
- *                      (Default: HMAC_DRBG_ENTROPY_LEN)
+ *                      (Default: given by the security strength, which
+ *                      depends on the hash used, see \c hmac_drbg_init() )
  *
  * \param ctx           HMAC_DRBG context
- * \param len           Amount of entropy to grab
+ * \param len           Amount of entropy to grab, in bytes
  */
 void hmac_drbg_set_entropy_len( hmac_drbg_context *ctx,
                                 size_t len );
 
+/**
+ * \brief               Set the reseed interval
+ *                      (Default: HMAC_DRBG_RESEED_INTERVAL)
+ *
+ * \param ctx           HMAC_DRBG context
+ * \param interval      Reseed interval
+ */
+void hmac_drbg_set_reseed_interval( hmac_drbg_context *ctx,
+                                    int interval );
+
 /**
  * \brief               HMAC_DRBG update state
  *
@@ -165,7 +180,7 @@ int hmac_drbg_reseed( hmac_drbg_context *ctx,
 /**
  * \brief               HMAC_DRBG generate random with additional update input
  *
- * Note: Automatically reseeds if reseed_counter is reached.
+ * Note: Automatically reseeds if reseed_counter is reached or PR is enabled.
  *
  * \param p_rng         HMAC_DRBG context
  * \param output        Buffer to fill
@@ -185,7 +200,7 @@ int hmac_drbg_random_with_add( void *p_rng,
 /**
  * \brief               HMAC_DRBG generate random
  *
- * Note: Automatically reseeds if reseed_counter is reached.
+ * Note: Automatically reseeds if reseed_counter is reached or PR is enabled.
  *
  * \param p_rng         HMAC_DRBG context
  * \param output        Buffer to fill

library/hmac_drbg.c

@@ -114,7 +114,8 @@ int hmac_drbg_reseed( hmac_drbg_context *ctx,
     /* 2. Update state */
     hmac_drbg_update( ctx, seed, seedlen );
 
-    /* 3. Reset reseed_counter (TODO) */
+    /* 3. Reset reseed_counter */
+    ctx->reseed_counter = 1;
 
     /* 4. Done */
     return( 0 );
@@ -145,6 +146,8 @@ int hmac_drbg_init( hmac_drbg_context *ctx,
     ctx->f_entropy = f_entropy;
     ctx->p_entropy = p_entropy;
 
+    ctx->reseed_interval = HMAC_DRBG_RESEED_INTERVAL;
+
     /*
      * See SP800-57 5.6.1 (p. 65-66) for the security strength provided by
      * each hash function, then according to SP800-90A rev1 10.1 table 2,
@@ -187,6 +190,14 @@ void hmac_drbg_set_entropy_len( hmac_drbg_context *ctx, size_t len )
     ctx->entropy_len = len;
 }
 
+/*
+ * Set reseed interval
+ */
+void hmac_drbg_set_reseed_interval( hmac_drbg_context *ctx, int interval )
+{
+    ctx->reseed_interval = interval;
+}
+
 /*
  * HMAC_DRBG random function with optional additional data (10.1.2.5)
  */
@@ -200,9 +211,10 @@ int hmac_drbg_random_with_add( void *p_rng,
     size_t left = out_len;
     unsigned char *out = output;
 
-    /* 1. Check reseed counter (TODO) and PR */
+    /* 1. Check reseed counter and PR */
     if( ctx->f_entropy != NULL &&
-        ctx->prediction_resistance == HMAC_DRBG_PR_ON )
+        ( ctx->prediction_resistance == HMAC_DRBG_PR_ON ||
+          ctx->reseed_counter > ctx->reseed_interval ) )
     {
         if( ( ret = hmac_drbg_reseed( ctx, additional, add_len ) ) != 0 )
             return( ret );
@@ -229,7 +241,8 @@ int hmac_drbg_random_with_add( void *p_rng,
     /* 6. Update */
     hmac_drbg_update( ctx, additional, add_len );
 
-    /* 7. Update reseed counter (TODO) */
+    /* 7. Update reseed counter */
+    ctx->reseed_counter++;
 
     /* 8. Done */
     return( 0 );